There are four puzzles which you can solve that will unlock the different forms of Unown in Gold, Silver, Crystal, HeartGold and SoulSilver. In the back of each room there will be a text written in Unown language. If you solve the quick puzzle you will get four items from each room. Here are the things you need to solve the four back room puzzles: Use Flash, use an Escape Rope, use a Water Stone (in HeartGold and Soulsilver) or have it in your bag (in Gold, Silver, and Crystal), and have Ho-oh first in your party.

If you tune your Pokegear radio to the right station, you can hear a strange radio transmission. It will be like a song or some sort of tune. If you have ever seen the YouTube video for the Pokemon creepypasta "Pokemon Lost Silver", you'd see that the radio transmission as it was in G/S is quite eerie when played backwards. (Just a little tidbit.)﻿
In HeartGold and SoulSilver (and Crystal, I believe), you will receive an UNOWN Report to record the kinds of Unown and extra info from the ruins of alph. "Alph" is short for "alphabet"

There are 28 kinds of Unown, 26 look like the alphabet letters and the other two look like the question mark and exclamation points.